1. Field of the Invention
This invention relates to a method and a detector for detecting arc faults and to circuit breakers incorporating such an arc fault detector and utilizing such a method. In particular, it relates to detecting arc faults in ac electrical power systems including aircraft and other ac electrical power systems which operate at a higher frequency such as 400 Hz than the 50 or 60 Hz of the typical electric power distribution system.
2. Background Information
Recent attention to the wiring problems associated with xe2x80x9caging aircraftxe2x80x9d has highlighted the deficiencies of the circuit protective devices designed to limit the wire damage and fire hazard associated with an electrical fault on an aircraft. The protection device most commonly used consists of an overload responsive trip mechanism such as a RMS current responsive bimetal. The bimetal has a resistance which dissipates energy in the form of heat at a rate which is a function of this resistance multiplied by the RMS current squared causing it to deflect. At a certain deflection, a spring powered operating mechanism is unlatched causing the circuit interrupters"" current carrying contacts to separate. This opens or deenergizes the circuit and causes the fault current to cease. The spring is recharged and latched by resetting the circuit interrupter. This type of overload protection does an excellent job of protecting wires and their insulation from degradation and damage due to a current induced over-temperature condition.
Aircraft circuit breakers typically employ only overload protection. While overcurrent protection in the form of an instantaneous trip function at a given value of load current is possible, such protection is normally not provided on aircraft circuit breakers because of concern that such protection can be susceptible to unwanted or nuisance tripping.
A fault current that flows between a conductor and a neutral or ground conductor when an arcing condition develops due to an insulation failure or breakdown is not continuous but is intermittent or sputtering in nature. An example of a waveform of such an arc fault is show in FIG. 1. The arc voltage of about 30 to 60 volts opposes the instantaneous line value. This results in four characteristics that help define the arcing current waveform. First, the arc current is zero for a period each half cycle before the arc can be restruck after a current zero. Second, the arc may not restrike at all or only after a long delay so that there are missing half cycles. Third, the resulting peak current is reduced below that which would flow if a xe2x80x9cboltedxe2x80x9d fault occurred at the same point in the electrical system. And finally, fourth, the current each half cycle will go to zero before voltage zero. These four characteristics are evident in the arc current waveform shown in FIG. 1. The intermittent nature of the current waveform reduces the RMS or bimetal-heating value below that required to trip the typical aircraft circuit breaker.
In order to develop a technique for detecting dangerous arc faults, the characteristic waveform or signature of safe current waveforms that exist on an aircraft must be analyzed so that the arc fault can be reliably distinguished from a safe current. The two most common loads on an aircraft are electronic and motor.
The starting current of a typical motor is shown in FIG. 2. As can be seen, the current does not exhibit the zero current crossing characteristics of an arcing current. The current tends to be high during the starting period and then decays to a smaller value as the motor reaches normal operating speed.
The normal in-rush charging current that exists when an electronic load is energized exhibits two of the four characteristics of a dangerous arcing current as can be seen in FIG. 3. Typically, the electronic load has a power supply which consists of a simple full wave current rectifier followed by a dc storage capacitor. The capacitor voltage opposes the source voltage each half cycle just as the arc voltage does, and thus the period of zero current around current zero crossing is the same for each waveform. The amplitude of the arcing current can be greater or less than the in-rush current, and thus the peak value of current can only be used to protect against very high arcing currents. Compared to an arcing current, the electronic in-rush current has no missing half cycles and decays each half cycle.
Other aircraft loads consist of resistive types like heaters and incandescent lamps, inductive loads such as florescent lamp ballasts, and possibly ac capacitive input loads. All of these loads have a zero current crossing much like that for the motor load, hence there is no period of zero current. Both the inductive and capacitive loads have a very large transient initial peak half cycle of current.
Much attention has been directed toward arc fault protection in residential electric power distribution systems. Generally, one of two techniques is employed. In the first, the high frequency characteristics of arc fault waveforms are exploited. These arc fault detectors look at such characteristics as the high frequency noise, the presence of zero current and missing half cycles. The second approach looks for the step increases in current resulting when an arc is struck. The arc fault detector disclosed in U.S. Pat. No. 5,691,869 utilizes a bandpass filter to generate a pulse having an amplitude which is a function of the magnitude of the step increase in voltage caused by striking of the arc. The magnitude of these pulses, or at least a portion above a selected threshold, is accumulated. When the accumulated value reaches a certain level, an arc fault indication is generated. The accumulated value is time attenuated so that pulses must occur with sufficient frequency and magnitude to reach the selected level.
The characteristics of aircraft electrical systems do not make it possible to employ the arc fault circuit breakers developed for residential electric power distribution systems. First, aircraft ac systems typically operate at 400 Hz as opposed to the 50 or 60 Hz of residential power systems. At 400 Hz, the quiescent periods in the arc current are much shorter. In addition, the waveforms are much steeper and therefore it is more difficult to distinguish the normal current rise from the step increases caused by an arc. Most importantly, aircraft electrical systems utilize small gauge wires which can be very long and therefore have substantial resistance. As a result, a load close to the generator can draw a normal current that far exceeds a fault current on a remote part of the wiring. More importantly, a fault current may draw a couple of thousand amps if close to the generator but only perhaps 30 or 40 amps if in a remote section of the wiring. Thus, it is not practical to rely on the magnitude of current in an aircraft electrical system to distinguish an arc fault.
There is a need therefore for an improved arc fault detector and method of arc detection for ac electrical power systems including aircraft ac electrical systems and other electrical systems operating at frequencies above the conventional 50 to 60 Hz and typically in the range of 400 Hz.
This need and others are satisfied by the invention which is directed to an arc fault detector for ac electrical power systems in general and is particularly suitable for an aircraft ac electrical system or other ac systems operating at higher frequencies such as 400 Hz. The arc fault detector of the invention includes a current detector detecting the ac current flowing in the electrical circuit, and a processor which generates a cumulative sum of amounts by which the ac current in each most recent cyclic interval exceeds the current in the immediately preceding half cycle in absolute magnitude. An arc fault indication is generated when this cumulative sum reaches a selected level. Preferably, the cumulative sum is time attenuated and the arc indication is generated when the time attenuated cumulative sum reaches a selected level. The current magnitude which is monitored can be the average magnitude or the RMS magnitude but is preferably the peak magnitude. The processor only adds the calculated differential to the time attenuated cumulative sum for cyclic intervals in which the current exceeds that for the immediately preceding cyclic interval in absolute magnitude by a selected amount after a first cyclic interval in which the current exceeds a selected arming magnitude. Hence, it takes a cyclic interval with current of a magnitude above the selected magnitude to arm the system. The processor terminates adding to the attenuated cumulative sum when the sum attenuates to a predetermined minimum level. At this point, the system is disarmed and the cumulative sum is cleared. In a preferred embodiment of the invention, the cyclic intervals comprise half cycles of the ac current.
The current detector includes a rectifier and a peak detector generating the peak value of each half cycle of the rectified current.
The arc fault detector can also include a delayed trip. Where the arc occurs every half cycle, and the peaks have similar magnitude, it may take a long time to accumulate sufficient differentials from cycle to cycle to reach the trip level. Accordingly, as another aspect of the invention, a short delay function is provided by short delay means which accumulate a time attenuated short delay accumulation of peak values of current for cyclic intervals (e.g., half cycles) in which the peak current is above a short delay threshold.
The invention also embraces a circuit breaker incorporating the above described arc fault detector. It further includes a method of arc fault detection which comprises the steps of: monitoring the magnitude of half cycles of the ac current, comparing this magnitude to a selected magnitude, for each half cycle exceeding the selected magnitude after a first half cycle exceeding that magnitude generating a cumulative sum of differences by which the magnitude exceeds the magnitude of an immediately preceding half cycle. The method further includes reducing the cumulative sum by a selected function of time and generating an arc fault indication when the cumulative sum reaches a preset value. In addition, the method includes clearing the cumulative sum when it attenuates to a selected minimum value. The method can also include generating a delayed trip as a time attenuated accumulation of peak values which exceed a threshold value.